Isometric exercise is an important component of most daily activities, yet much of the research in this area is purely descriptive and assumes intrinsic physiological differences between it and dynamic exercise. The proposed study examines cardiovascular regulation during isometric exercise by definition of (a) local metabolic state reflected by percent maximum voluntary contraction (%MVC); (b) total metabolic demand (mass of active muscle); and (c) cardiovascular responses. This will be done by determining (1) the relationship of MVC to local metabolic state and local blood flow, (2) the relationship of MVC and total mass of active muscle to central CV changes, (3) the role of baroreflexes during isometric exercise, and (4) whether differences in local blood flow and conductance account for the systemic CV differences of isometric vs. dynamic exercise. Several techniques will be utilized: local metabolic state will be assessed by femoral vein flow determination (thermodilution) and by analysis of venous effluent blood samples and muscle biopsies; central cardiorespiratory state by measures of cardiac output and oxygen consumption (mass spec C2H2 rebreathing); blood pressure, heart rate, and ventricular volume and contractility (echocardiography). The central responses to a particular local metabolic state produced by an isometric contraction will be determined during dynamic exercise and modified to create similar local conditions by variations in local blood flow and the energy expense. Responses to isometric and dynamic exercise can then be compared on the basis of the local metabolic environment. This will permit the formulation of a single set of regulatory mechanisms which will account for the responses seen in both types of exercise. Such a framework could prove useful in the study of other cardiovascular problems which have as a common basis alterations in local metabolic state and flow such as peripheral vascular disease.